Biomass oxygen/steam gasification in a pressurized bubbling fluidized bed: Agglomeration behavior

In this study, the anti-agglomeration abilities of Ca- and Mg-containing bed materials, including dolomite and magnesite, in a pressurized bubbling fluidized bed gasifier using pine pellets and birch chips as feedstock, is investigated. The most typical bed material—silica sand—was also included as a reference for comparison. The sustainability of the operation was evaluated via analyzing the temperatures at different levels along the bed height. During the performances, the aim was to keep the temperature at the bottom zone of the reactor at around 870°C. However, the success highly depends on the bed materials used in the bed and the temperature can vary significantly in case of agglomeration or bad mixing of bed materials and char particles. Both Glanshammar and Sala dolomites performed well with no observed agglomeration tendencies. In case of magnesite, the bed exhibited a high agglomeration tendency. Silica sand displayed the most severe agglomeration among all bed materials, even when birch chips with a low silica content was fed at a relatively low temperature. The solid samples of all the bed materials were inspected by light microscopy and Scanning Electron Microscopy (SEM). The Energy Dispersive Spectroscopy (EDS) detector was used to detect the elemental distribution in the surface. The crystal chemical structure was analyzed using X-ray Diffraction (XRD). Magnesite agglomerates glued together by big molten ash particles. There was no coating layer detected on magnesite particles at bed temperatures – below 870°C. But when the temperature was above 1000°C, a significant amount of small molten ash particles was deposited on the magnesite particles, indicating a pronounced tendency for formation of a coating layer in case of long-term operation. An increasing trend of Si on the surface of dolomite particles was observed. Simultaneously, potassium deposition on the surface is not obvious. The analyses, based on the XRD diffraction and the K2O–SiO2–MgO and K2O–SiO2–CaO ternary diagrams, suggest that the observed decreases in the risks for agglomeration using dolomite, cannot be attributed to formation of alkali-containing compounds with higher melting points, but to the reaction between dolomite and silica, consuming a considerable portion of silicon and thus hinder the formation of low-melting potassium silicate, as well as its ability to stabilize the temperatures under pressurized conditions.